Multistage gear pump

ABSTRACT

A multistage gear pump for pressurizing fluid includes a housing, a drive shaft and a gear assembly. The drive shaft is rotatably supported in the housing. The gear assembly is disposed in the housing. The gear assembly includes at least first and second gear trains. Each train has a pair of drive and driven gears that are engaged with each other. The drive gear is provided on the drive shaft and followed by the driven gear. The first gear train and the second gear train are arranged so that fluid sequentially passes therethrough as the drive shaft rotates. A theoretical discharge capacity of the first gear train is larger than that of the second gear train.

[0001] The present invention relates to a multistage gear pump thatpressurizes suctioned fluid with gear trains to discharge thepressurized fluid.

[0002] Generally, there is a multistage gear pump that pressurizes fluidwith gear trains (e.g. Japanese Unexamined Patent Publication No.2001-140770). FIG. 7 is a longitudinal cross-sectional view of amultistage gear pump 81 (hereinafter referred to as a pump) disclosed inJapanese Unexamined Patent Publication No. 2001-140770. The pump 81includes a first-stage rotary gear 84 having a pair of gears 82 and 83and a second-stage rotary gear 87 having a pair of gears 85 and 86. Therotary gears 84 and 87 are arranged in adjacent to each other in anaxial direction. The gears 82 and 85 are supported by a drive shaft 88while the gears 83 and 86 are supported by a driven shaft 89.

[0003] In the pump 81, when the drive shaft 88 rotates, the gears 82 and85 rotate, and the gears 83 and 86 respectively rotate to follow thegears 82 and 85. At the time, the pump 81 sucks liquid, and the suckedliquid flows to the first-stage rotary gear 84 and is pressurized. Thepressurized liquid flows to the second-stage rotary gear 87 through apassage 90 that interconnects the first-stage rotary gear 84 with thesecond-stage rotary gear 87. The pressurized liquid is furtherpressurized at the rotary gear 87 and is discharged at a predeterminedhigh-pressure state.

[0004] For example, it is assumed that dimethylether (DME) is utilizedas the liquid. Since the DME easily leaks due to the low viscosity, theDME leaks out from gear portions of the rotary gears 84 and 87 in anoperating state of the pump 81. Therefore, an amount of the pressurizedDME sent from the first-stage rotary gear 84 to the second-stage rotarygear 87, that is, an actual amount of the DME discharged from thefirst-stage rotary gear 84 is smaller than a discharge capacity (atheoretical value) of the first-stage rotary gear 84 due to the aboveleakage.

[0005] Therefore, when the facewidth of the rotary gear 84 is equal tothat of the rotary gear 87 as disclosed in Japanese Unexamined PatentPublication No. 2001-140770, that is, when the discharge capacity of thefirst-stage rotary gear 84 is equal to that of the second-stage rotarygear 87, the actual discharge capacity of the first-stage rotary gear 84is insufficient for the discharge capacity of the second-stage rotarygear 87. Since the DME has the nature of a relatively high vaporpressure (a relatively high volatility), the pressure of the DME fallsbelow the vapor pressure and vaporizes. In the result, the DME cannot bepressurized to a predetermined pressure.

SUMMARY OF THE INVENTION

[0006] The present invention provides a multistage gear pump that canensure pressurization characteristics for fluid.

[0007] In accordance with the present invention, a multistage gear pumpfor pressurizing fluid includes a housing, a drive shaft and a gearassembly. The drive shaft is rotatably supported in the housing. Thegear assembly is disposed in the housing. The gear assembly includes atleast first and second gear trains. Each train has a pair of drive anddriven gears that are engaged with each other. The drive gear isprovided on the drive shaft and followed by the driven gear. The firstgear train and the second gear train are arranged so that fluidsequentially passes therethrough as the drive shaft rotates.Atheoretical discharge capacity of the first gear train is larger thanthat of the second gear train.

[0008] The present invention also provides a multistage gear pump forpressurizing fluid. The multistage gear pump includes a housing, a driveshaft, a driven shaft and a gear assembly. The drive shaft is rotatablysupported in the housing. The driven shaft is rotatably supported in thehousing. The gear assembly is disposed in the housing. The gear assemblyincludes at least first and second gear trains. Each train has a pair ofdrive and driven gears that are engaged with each other. The drive gearis provided on the drive shaft and followed by the driven gear. Thefirst gear train and the second gear train are arranged so that fluidsequentially passes therethrough as the drive shaft rotates. One of thedriven gears is formed with the driven shaft so as to rotate integrallywith the driven shaft. The rest of the driven gears are assembled to thedriven shaft so as to rotate relative to the driven shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The features of the present invention that are believed to benovel are set forth with particularity in the appended claims. Theinvention together with objects and advantages thereof, may best beunderstood by reference to the following description of the presentlypreferred embodiments together with the accompanying drawings in which:

[0010]FIG. 1A is a longitudinal cross-sectional view of a three-stagegear pump according to a first preferred embodiment;

[0011]FIG. 1B is a partially enlarged longitudinal cross-sectional viewof the three-stage gear pump according to the first preferredembodiment;

[0012]FIG. 2 is a cross-sectional view of the three-stage gear pumptaken along the line II-II in FIG. 1A;

[0013]FIG. 3 is a cross-sectional view of the three-stage gear pumptaken along the line III-III in FIG. 1A;

[0014]FIG. 4 is a schematic view of a fuel supply system according tothe first preferred embodiment;

[0015]FIG. 5 is a partially enlarged longitudinal cross-sectional viewof the three-stage gear pump around an O-ring according to the firstpreferred embodiment;

[0016]FIG. 6 is a longitudinal cross-sectional view of a two-stage gearpump according to a second preferred embodiment; and

[0017]FIG. 7 is a longitudinal cross-sectional view of a multistage gearpump according to a prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, first and second preferred embodiments according tothe present invention will be described. The present invention isapplied to a multistage gear pump for use in a fuel supply system forsupplying fuel (dimethylether) to an engine as a drive source for avehicle.

[0019] Now the first preferred embodiment will be described. FIG. 4 is aschematic view of a fuel supply system. Dimethylether (DME) as fluid isstored in a tank 2 that is connected to a multistage gear pump 1 (athree-stage gear pump in the present preferred embodiment and referredto as a pump hereinafter) at the input side of the pump 1 through asuction pipe 3. An injection pump 5 is connected to the pump 1 at theoutput side of the pump 1 through a discharge pipe 4. An engine 6 isconnected to the injection pump 5 at the output side of the injectionpump 5. The pressurized DME is sent from the pump 1 to the injectionpump 5 which supplies the DME having a high pressure into the engine 6.

[0020]FIG. 1A is a longitudinal cross-sectional view of the pump 1. Whenthe pump 1 is installed in a vehicle, the left and right sides of thepump 1 in FIG. 1A respectively correspond to the upper and the lowersides of the pump 1. The pump 1 includes a casing 7 and a cover 9. Thecasing 7 has a cylindrical shape with a bottom. The cover 9 is securedto the casing 7 through a plurality of bolts 8. The casing 7 and thecover 9 constitute a housing of the pump 1. A motor 10 as a drive sourceand a gear assembly 11 are accommodated in the casing 7. The motor 10 issecured to the inner surface of the cover 9. The gear assembly 11 isfixed to the motor 10.

[0021] The motor 10 includes a motor housing 10 a, a stator 10 b and arotor 10 c. The stator 10 b has a coil that is arranged along the innercircumferential surface of the motor housing 10 a. The rotor 10 cincludes an iron core that is arranged so as to be surrounded by thestator 10 b. The rotor 10 c is integrally and rotatably secured to adrive shaft 12. A bearing 13 is arranged at the motor housing 10 a onthe upper side, and a bearing 14 is arranged in a bottom plate 22. Bothends of the drive shaft 12 are respectively supported by the bearings 13and 14. Namely, the drive shaft 12 is supported in the housingrotatably. The coil of the stator 10 b is connected to a terminal 15.When an external device supplies electric current to the coil throughthe terminal 15, the drive shaft 12 rotates due to the action ofelectromagnetic induction between the coil of the stator 10 b and theiron core of the rotor 10 c.

[0022] As shown in FIGS. 1A and 1B, the gear assembly 11 includes a baseblock 16, a connecting plate 17, a side plate 18, a connecting plate 19,a side plate 20, a connecting plate 21 and the bottom plate 22 in orderfrom a side of the motor 10. The drive shaft 12 extends through the baseblock 16 and these plates 17 through 22. In this state, the base block16 and these plates 17 through 22 are integrally fixed to each other bythreading a plurality of bolts 23 (shown in FIGS. 2 and 3) thereinto. Aplurality of bolts 24 is threaded into the motor housing 10 a through aflange 16 a of the base block 16. Therefore, the gear assembly 11 isfixed to the motor 10.

[0023] The drive shaft 12 extends through the base 16 and the plates 17through 22 that constitute the gear assembly 11, and the lower end ofthe drive shaft 12 is supported by the bottom plate 22 through thebearing 14. A groove 12 a is formed on the outer circumferential surfaceof the drive shaft 12 at the lower side. A key 25 having a rectangularsolid shape is fitted in the groove 12 a.

[0024] Three gears 26 through 28 are provided on the drive shaft 12 inorder from the lower side along an axial direction of the drive shaft12. Gear tooth 26 a through 28 a are respectively formed on the outercircumferential surfaces of the gears 26 through 28. Key seats 26 bthrough 28 b are respectively formed on the inner circumferentialsurfaces of the gears 26 through 28. The key 25 is locked in the keyseats 26 b through 28 b. Therefore, the gears 26 through 28 rotateintegrally with the drive shaft 12. The same material is used for theside plates 18 and 20. Incidentally, the gears 26 through 28 are drivegear.

[0025] The gear assembly 11 accommodates a driven shaft 29 that isparallel to the drive shaft 12. The driven shaft 29 extends through thebase 16 and the plates 17 through 22 that constitute the gear assembly11. The upper end of the driven shaft 29 is supported by the base block16 through a bearing 30, and the lower end of the driven shaft 29 issupported by the bottom plate 22 through a bearing 31.

[0026] Three gears 32 through 34 are provided on the driven shaft 29 inorder from the lower side along an axial direction of the driven shaft29. Gear tooth 32 a through 34 a are respectively formed on the outercircumferential surfaces of the gears 32 through 34. The gear 32, whichis at the lowest side, is formed integrally with the driven shaft 29. Onthe other hand, the gears 33 and 34 respectively have through holes, andthe driven shaft 29 penetrates through the through holes of the gears 33and 34. Therefore, the gears 33 and 34 are assembled to the driven shaft29 so as to be rotatable relative to the driven shaft 29. The gears 26and 32 have a same facewidth h1, the gears 27 and 33 have a samefacewidth h2, and the gears 28 and 34 have a same facewidth h3. Thegears 26 through 28 are respectively engaged with the gears 32 through34. Incidentally, the gears 32 through 34 are driven gear.

[0027] A suction connecting portion 35 is connected to the outercircumferential surface of the casing 7. The suction pipe 3 extendedfrom the tank 2 is connected to the suction connecting portion 35. Thepump 1 sucks the DME in the tank 2 from a suction port 35 a in anoperating state of the pump 1. The pump 1 is a series-type pump thatpressurizes the sucked DME by passing the sucked DME through a pluralityof gear trains. Namely, the pump 1 passes the sucked DME through afirst-stage gear train 36 constituted of the gears 26 and 32, asecond-stage gear train 37 constituted of the gears 27 and 33 and athird-stage gear train 38 constituted of the gears 28 and 28sequentially, thereby pressurizing the sucked DME. As shown in FIG. 2, adischarge connecting portion 39 is connected to the outercircumferential surface of the casing 7. The pump 1 discharges thepressurized DME from a discharge port 39 a of the discharge connectingportion 39.

[0028] In a relationship between the first-stage gear train 36 and thesecond-stage gear train 37, the first-stage gear train 36 and thesecond-stage gear train 37 are respectively considered a low-pressurestage gear train or a first gear train and a high-pressure stage geartrain or a second gear train. In a relationship between the second-stagegear train 37 and the third-stage gear train 38, the second-stage geartrain 37 and the third-stage gear train 38 are respectively considered alow-pressure stage gear train and a high-pressure stage gear train. Thethird-stage gear train 38 is the highest-pressure stage gear train whosepressure is the largest among the gear trains 36 through 38.

[0029]FIG. 2 is a cross-sectional view of the pump 1 taken along theline II-II in FIG. 1A, and FIG. 3 is a cross-sectional view of the pump1 taken along the line III-III in FIG. 1A. As shown in FIG. 2, holes 21a and 21 b is respectively formed in the connecting plate 21 foraccommodating the gears 26 and 32. In the connecting plate 21, slightspace regions are provided on frontward and backward sides of the placewhere the gear 26 is engaged with the gear 32, thereby defining upstreamand downstream passages 40 and 41 as a passage for the DME. The upstreampassage 40 communicates with the suction port 35 a. Similarly, holes andupstream and downstream passages are formed in each of the connectedplates 17 and 19.

[0030] The drive shaft 12 is rotated by the drive of the motor 10 in adirection (clockwise) indicated by an arrow A in FIG. 2. The drivenshaft 29 is rotated in a direction indicated by an arrow B in FIG. 2 inaccordance with the rotation of the drive shaft 12 through thefirst-stage gear train 36. The DME is drawn into the inside of the pump1 due to the rotations of the drive shaft 12 and the driven shaft 29,and flows to the first-stage gear train 36 through the suction port 35 aand the upstream passage 40. Pump chambers 36 a are defined by theadjacent gear tooth 26 a of the gear 26 and the inner circumferentialsurface of the hole 21 a. Pump chambers 36 b are defined by the adjacentgear tooth 32 a of the gear 32 and the inner circumferential surface ofthe hole 21 b. The DME that reaches the first-stage gear train 36 issent toward the downstream passage 41 through the pump chambers 36 a and36 b.

[0031] As shown in FIG. 3, adjacent holes 20 a and 20 b are respectivelyformed 20 in the side plate 20 such that the drive shaft 12 and thedriven shaft 29 extend therethrough. The diameter of the hole 20 a isset to be larger than that of the drive shaft 12. Therefore, a clearanceis formed between the inner circumferential surface of the hole 20 a andthe outer circumferential surface of the drive shaft 12. The diameter ofthe hole 20 b is set to be larger than that of the driven shaft 29.Therefore, a clearance is formed between the inner circumferentialsurface of the hole 20 b and the outer circumferential surface of thedriven shaft 29.

[0032] A communication passage 43 is formed in the side plate 20 andinterconnects the downstream passage 41 at the first-stage gear train 36with an upstream passage 42 at the second-stage gear train 37. Thecommunication passage 43 includes first, second and third passages 43 athrough 43 c. The first passage 43 a extends in a radial direction ofthe pump 1. The second passage 43 b extends from the downstream passage41 at the first-stage gear train 36 in the axial direction andcommunicates with the first passage 43 a. The third passage 43 c extendsfrom the upstream passage 42 at the second-stage gear train 37 andcommunicates with the first passage 43 a.

[0033] Pump chambers 37 a are defined by the adjacent gear tooth 27 a ofthe gear 27 and the inner circumferential surface of the connectingplate 19. Pump chambers 37 b are defined by the adjacent gear tooth 33 aof the gear 33 and the inner circumferential surface of the connectingplate 19. The DME that reaches 20 the second-stage gear train 37 is sentto the third-stage gear train 38 through the pump chambers 37 a and 37b. Also, pump chambers 38 a are defined by the adjacent gear tooth 28 aof the gear 28 and the inner circumferential surface of the connectingplate 17. Pump chambers 38 b are defined by the adjacent gear tooth 34 aof the gear 34 and the inner circumferential surface of the connectingplate 17. The DME that reaches the third-stage gear train 38 is sent tothe discharge port 39 a through the pump chambers 38 a and 38 b.

[0034] As shown in FIG. 1B, O-rings 48 a through 48 f are respectivelyarranged in the gear assembly 11 for ensuring sealing between the pumpchambers 36 a through 38 a and 36 b through 38 b and the interior of thecasing 7. The O-rings 48 a through 48 f are arranged so as to surroundthe drive shaft 12 and the driven shaft 29. There is an internal chamber51 around the drive shaft 12 in the gear assembly 11. Seal rings 49 athrough 49 f are respectively arranged in the gear assembly 11 forensuring sealing between the pump chambers 36 a through 38 a and theinternal chamber 51. The seal rings 49 a through 49 f are arranged so asto surround the drive shaft 12.

[0035] There is an internal chamber 52 around the driven shaft 29 in thegear assembly 11. Seal rings 50 a through 50 f are respectively arrangedin the gear assembly 11 for ensuring sealing between the pump chambers36 b through 38 b and the internal chamber 52. The seal rings 50 athrough 50 f are arranged so as to surround the driven shaft 29. Nitrilerubber is used for the material of the O-rings 48 a through 48 f.Polytetrafluoroethylene is used as the material of the seal rings 49 athrough 49 f and 50 a through 50 f.

[0036] Sealing state of the seal ring 49 a will be described withreference to FIG. 5. The seal rings 49 b through 49 f and 50 a through50 f also behave in the same manner as the seal ring 49 a. Since the DMEin the pump chamber 36 a is pressurized more than that in the internalchamber 51, the DME flows from the pump chamber 36 a into a groove 53that accommodates the seal ring 49 a as shown by an arrow C in FIG. 5.Due to the flow of the DME, the seal ring 49 a moves toward the sides ofgear 26 and the drive shaft 12 and contacts the side surface of the gear26 and the undersurface of the groove 53, thereby creating sealing.

[0037] As shown in FIG. 1B, the internal chamber 51 is divided intofirst, second and third chambers 51 a through 51 c in order from thelower side of the drive shaft 12 by the gears 26 through 28. Thechambers 51 a through 51 c communicate with each other through a slightclearance between the key 25 and the groove 12 a. A return passage (notshown) is formed in the gear assembly 11 so as to interconnect the thirdchamber 51 c with the suction port 35 a.

[0038] Since the DME has a low viscosity, the DME (vapor liquid) leaksout from the pump chambers 36 a through 38 a into the internal chamber51 via the seal rings 49 a through 49 f. When the high-pressure DMEleaks out into the internal chamber 51 as mentioned above, the pressurein the internal chamber 51 increases due to the high-pressure DME.Therefore, it is thought that thrust load acts on the drive shaft 12 inthe axial direction. However, the leaking DME in the internal chamber 51is returned to the suction port 35 a through the return passage.Therefore, the pressure in the internal chamber 51 is substantiallyequal to a suction pressure, and the thrust load does not act on thedrive shaft 12.

[0039] As shown in FIG. 3, sealing members 44 and 45 respectively sealsboth ends of the communication passage 43. A relief valve 46 as a valvemeans is provided at the end portion of the first passage 43 a at a sideof the second passage 43 b of the communication passage 43 in the sideplate 20. A hole 46 b is formed in a valve chamber 46 a of the reliefvalve 46 so as to communicate with the discharge port 39 a. The reliefvalve 46 includes a valve body 46 c and an urging spring 46 d. The valvebody 46 c has a spherical shape. The urging spring 46 d urges the valvebody 46 c in a direction in which the relief valve 46 closes.

[0040] Similarly to the side plate 20, a communication passage 47 isformed in the side plate 18 as shown in FIG. 1B. The communicationpassage 47 interconnects a downstream passage at the second-stage geartrain 37 with an upstream passage at the third-stage gear train 38. Arelief valve (not shown) is also provided in the side plate 18. Adownstream passage (not shown) at the third-stage gear train 38 isformed in the base block 16 and communicates with the dischargeconnecting portion 39. A flow path includes the suction port 35 a, theupstream passages 40 and 42, the upstream passage at the third-stagegear train 38, the downstream passage 41, the downstream passages at thesecond and third gear trains 37 and 38, the communication passages 43and 47, the pump chambers 36 a through 38 a and 36 b through 38 b, anddischarge port 39 a.

[0041] When a pressure of the DME discharged into the communicationpassage 43 is lower than a predetermined value, the valve body 46 ccontacts a valve seat 46 e due to the spring force of the urging spring46 d to close the relief valve 46. Therefore, substantially all of theDME discharged into the communication passage 43 is sent to thesecond-stage gear train 37. On the other hand, when the pressure of theDME discharged into the communication passage 43 is larger than thepredetermined value, the valve body 46 c is pushed away from the valveseat 46 e against the urging spring 46 d. The DME is directly dischargedinto the discharge port 39 a through the hole 46 b by bypassing thesecond-stage and third-stage gear trains 37 and 38.

[0042] As shown in FIG. 1A, a pipe connecting portion 54 is connected tothe cover 9. A pipe 55 for leak is extended from the tank 2 and isconnected to the pipe connecting portion 54 as shown in FIG. 4. A port56 for leak is formed in the pipe connecting portion 54 so as tointerconnect the internal space of the motor 10 with the outside of thepump 1. A drain hole 57 is formed in the sidewall of the motor housing10 a so as to interconnect the internal space of the motor 10 with aspace that is inside the casing 7 and outside the motor housing 10 a.

[0043] In the pump 1 that seals a shaft therein, sliding portions of themotor 10 and the gear assembly 11, for example, the gears 26 through 28and 32 through 34, the drive shaft 12 and the driven shaft 29 generateheat. Due to the heat, the DME leaking out from the gear trains 36through 38 vaporizes. It is thought that the vaporizing DME is stored inthe internal space of the motor housing 10 a and the space that isinside the casing 7 and outside the motor housing 10 a.

[0044] However, the DME gas in the internal space of the motor housing10 a is returned from the port 56 into the tank 2 through the pipe 55.The DME gas in the space that is inside the casing 7 and outside themotor housing 10 a flows into the internal space of the motor housing 10a through the drain hole 57 and is returned from the port 56 into thetank 2 through the pipe 55.

[0045] As shown in FIG. 4, a feedback pipe 58 connects the pipe 55 tothe injection pump 5. The redundant DME that is not injected and thatremains in the injection pump 5 is returned into the tank 2 through thefeedback pipe 58 and the pipe 55.

[0046] As shown in FIG. 1B, a theoretical discharge capacity of thefirst-stage gear train 36 is determined as D1 in one rotation of thegears 26 and 32. The fluid leakage is not considered into thetheoretical discharge capacity D1 of the first-stage gear train 36. Anactual discharge capacity S1 of the first-stage gear train 36 is smallerthan the theoretical discharge capacity D1 of the first-stage gear train36 due to the leakage of the DME, which is caused due to the lowviscosity of the DME. Similarly, a theoretical discharge capacity of thesecond-stage gear train 37 is determined as D2. An actual dischargecapacity S2 of the second-stage gear train 37 is smaller than thetheoretical discharge capacity D2 of the second-stage gear train 37.Also, a theoretical discharge capacity of the third-stage gear train 38is determined as D3. An actual discharge capacity S3 of the third-stagegear train 38 is smaller than the theoretical discharge capacity D3 ofthe third-stage gear train 38.

[0047] When the DME is pressurized at the gear trains 36 through 38, theDME leaks out from the gear trains 36 through 38. Therefore, the actualdischarge capacities S1 through S3 of the gear trains 36 through 38,that is, actual volumes of the DME discharged from the gear trains 36through 38 respectively reduce in comparison to the theoreticaldischarge capacities D1 through D3. In the pump 1 of the presentpreferred embodiment, the leakage of the DME is suppressed by utilizingthe above sealing structure (the O-rings 48 a through 48 f and the sealrings 49 a through 49 f and 50 a through 50 f). Consequently, the actualdischarge capacity S1 of the first-stage gear train 36 is determined tobe 70 to 80 percentages of the theoretical discharge capacity D1, andthe actual discharge capacity S2 of the second-stage gear train 37 isdetermined to be 70 to 80 percentages of the theoretical dischargecapacity D2.

[0048] As mentioned in the above background, when the theoreticaldischarge capacity D1 of the first-stage gear train 36 is equal to thetheoretical discharge capacity D2 of the second-stage gear train 37, theactual discharge capacity S1 of the first-stage gear train 36 isinsufficient for the theoretical discharge capacity D2 of thesecond-stage gear train 37. Therefore, the DME cannot be pressurized toa predetermined pressure. Also, when the theoretical discharge capacityD2 of the second-stage gear train 37 is equal to the theoreticaldischarge capacity D3 of the third-stage gear train 38, the actualdischarge capacity S2 of the second-stage gear train 37 is insufficientfor the theoretical discharge capacity D3 of the third-stage gear train38. Therefore, the DME cannot be pressurized to a predeterminedpressure.

[0049] In the present preferred embodiment, the theoretical dischargecapacity D2 of the second-stage gear train 37 is set to be smaller thanthe theoretical discharge capacity D1 of the first-stage gear train 36.Also, the theoretical discharge capacity D3 of the third-stage geartrain 38 is set to be smaller than the theoretical discharge capacity D2of the second-stage gear train 37. Therefore, it is suppressed that theactual discharge capacity S1 of the first-stage gear train 36 isinsufficient for the theoretical discharge capacity D2 of thesecond-stage gear train 37. Also, it is suppressed that the actualdischarge capacity S2 of the second-stage gear train 37 is insufficientfor the theoretical discharge capacity D3 of the third-stage gear train38. Therefore, the DME is difficult to vaporize at the second and thirdgear trains 37 and 38.

[0050] The theoretical discharge capacities D1 through D3 of the geartrains 36 through 38 are respectively proportional to the facewidths h1through h3 of the gear trains 36 through 38. The theoretical dischargecapacities D1 through D3 of the gear trains 36 through 38 arerespectively determined by the facewidths h1 through h3 of the geartrains 36 through 38. Namely, the facewidth h2 of the second-stage geartrain 37 is set to be smaller than the facewidth h1 of the first-stagegear train 36, and the facewidth h3 of the third-stage gear train 38 isset to be smaller than the facewidth h2 of the second-stage gear train37.

[0051] In the relationship between the first-stage gear train 36 and thesecond-stage gear train 37, the facewidth h2 of the second-stage geartrain 37 as the high-pressure stage gear train is set to be smaller thanthe facewidth h1 of the first-stage gear train 36 as the low-pressurestage gear train. Therefore, the theoretical discharge capacity D2 ofthe second-stage gear train 37 is set to be smaller than the theoreticaldischarge capacity D1 of the first-stage gear train 36. The first andsecond gear trains 36 and 37 are respectively constituted of the gears26 and 32 and the gears 27 and 33. Namely, the gears 26, 27, 32 and 33have the same shapes and the same dimensions with respect to end facesaround the gear tooth 26 a, 27 a, 32 a and 33 a. Therefore, the shapesand the dimensions of the gear tooth 26 a and 32 a of the gears 26 and32 are same as those of the gear tooth 27 a and 33 a of the gears 27 and33 except for the facewidths h1 and h2. The relationship between thesecond and third gear trains 37 and 38 is same.

[0052] Meanwhile, assuming that the theoretical discharge capacity D2 ofthe second-stage gear train 37 is set to be excessively smaller than thetheoretical discharge capacity D1 of the first-stage gear train 36, anexcessive amount of the DME is sent from the first-stage gear train 36to the second-stage gear train 37. Leak rate of the DME at thesecond-stage gear train 37 is excessive. Also, assuming that thetheoretical discharge capacity D3 of the third-stage gear train 38 isset to be excessively smaller than the theoretical discharge capacity D2of the second-stage gear train 37, leak rate of the DME at thethird-stage gear train 38 is similarly excessive.

[0053] In the present preferred embodiment, the theoretical dischargecapacity D2 of the second-stage gear train 37 is equal to the actualdischarge capacity S1 of the first-stage gear train 36. Also, thetheoretical discharge capacity D3 of the third-stage gear train 38 isequal to the actual discharge capacity S2 of the second-stage gear train37. Therefore, the DME is not excessively sent from the first-stage geartrain 36 to the second-stage gear train 37. The leak rate of the DME atthe second-stage gear train 37 is not relatively large. Also, the DME isnot excessively sent from the second-stage gear train 37 to thethird-stage gear train 38, and the leak rate of the DME at thethird-stage gear train 38 is not relatively large.

[0054] The theoretical discharge capacity D2 of the second-stage geartrain 37 is equal to the actual discharge capacity S1 of the first-stagegear train 36. This equal includes a slight tolerance that obtainssubstantially the same effect. A maximum tolerance of the slighttolerance is 10 percentages of the theoretical discharge capacity D1 ofthe first-stage gear train 36. To obtain a more advantageous effect, themaximum tolerance is 5 percentages of the theoretical discharge capacityD1 of the first-stage gear train 36. Also, the theoretical dischargecapacity D3 of the third-stage gear train 38 is equal to the actualdischarge capacity S2 of the second-stage gear train 37. This equal alsoincludes a slight tolerance. A maximum tolerance of the slight toleranceis 10 percentages of the theoretical discharge capacity D2 of thesecond-stage gear train 37. To obtain the more advantageous effect, themaximum tolerance is 5 percentages of the theoretical discharge capacityD2 of the second-stage gear train 37.

[0055] Therefore, assuming that the actual discharge capacity S1 of thefirst-stage gear train 36 is 70 percentages of the theoretical dischargecapacity D1 of the first-stage gear train 36, the theoretical dischargecapacity D2 of the second-stage gear train 37 is set to be 60 to 80percentages of the theoretical discharge capacity D1 of the first-stagegear train 36. To obtain the more advantageous effect, the theoreticaldischarge capacity D2 of the second-stage gear train 37 is set to be 65to 75 percentages of the theoretical discharge capacity D1 of thefirst-stage gear train 36. Namely, the facewidth h2 of the second-stagegear train 37 is set to be 60 to 80 percentages of the facewidth h1 ofthe first-stage gear train 36. To obtain the more advantageous effect,the facewidth h2 of the second-stage gear train 37 is set to be 65 to 75percentages of the facewidth h1 of the first-stage gear train 36.

[0056] Therefore, assuming that the actual discharge capacity S2 of thesecond-stage gear train 37 is 70 percentages of the theoreticaldischarge capacity D2 of the second-stage gear train 37, the theoreticaldischarge capacity D3 of the third-stage gear train 38 is set to be 60to 80 percentages of the theoretical discharge capacity D2 of thesecond-stage gear train 37. To obtain the more advantageous effect, thetheoretical discharge capacity D3 of the third-stage gear train 38 isset to be 65 to 75 percentages of the theoretical discharge capacity D2of the second-stage gear train 37. Namely, the facewidth h3 of thethird-stage gear train 38 is set to be 60 to 80 percentages of thefacewidth h2 of the second-stage gear train 37. To obtain the moreadvantageous effect, the facewidth h3 of the third-stage gear train 38is set to be 65 to 75 percentages of the facewidth h2 of thesecond-stage gear train 37.

[0057] Next, action of the pump 1 as constructed above will bedescribed. When the motor 10 is energized and the drive shaft 12rotates, the pump 1 sucks the DME in the tank 2 through the suction pipe3. The sucked DME is sent to the suction side of the first-stage geartrain 36 and is pressurized by flowing through the pump chambers 36 aand 36 b at the first-stage gear train 36. At the time, a predeterminedamount of the DME leaks out from the pump chambers 36 a and 36 b to theoutside of the pump chambers 36 a and 36 b. The DME having theactual:volume, which is decreased by the predetermined amount,corresponding to the actual discharge capacity S1 flows to thesecond-stage gear train 37 through the communication passage 43.

[0058] The DME is pressurized further by flowing through the pumpchambers 37 a and 37 b at the second-stage gear train 37. At the time, apredetermined amount of the DME leaks out from the pump chambers 37 aand 37 b to the outside of the pump chambers 37 a and 37 b. The DMEhaving the actual volume, which is decreased by the predeterminedamount, corresponding to the actual discharge capacity S2 flows to thethird-stage gear train 38 through the communication passage 47. The DMEis pressurized further by flowing through the pump chambers 38 a and 38b. A predetermined amount of the DME leaks out from the pump chambers 38a and 38 b to the outside of the pump chambers 38 a and 38 b. The DMEhaving the actual volume, which is decreased by the predeterminedamount, corresponding to the actual discharge capacity S3 is suppliedinto the injection pump 5 through the discharge port 39 a and thedischarge pipe 4.

[0059] Following advantageous effects are obtained in the presentpreferred embodiment.

[0060] (1-1) In the present preferred embodiment, the theoreticaldischarge capacity D2 of the second-stage gear train 37 is set to besmaller than the theoretical discharge capacity D1 of the first-stagegear train 36. Also, the theoretical discharge capacity D3 of thethird-stage gear train 38 is set to be smaller than the theoreticaldischarge capacity D2 of the second-stage gear train 37. Therefore, itis suppressed that the actual discharge capacity S1 of the first-stagegear train 36 is insufficient for the theoretical discharge capacity D2of the second-stage gear train 37. Also, it is suppressed that theactual discharge capacity S2 of the second-stage gear train 37 isinsufficient for the theoretical discharge capacity D3 of thethird-stage gear train 38. Consequently, the DME is difficult tovaporize at the second and third gear trains 37 and 38, andpressurization characteristics for the DME is ensured.

[0061] (1-2) When the DME vaporizes, cavitation occurs in the DME. Ascavitation bubbles burst, shock waves occur. Therefore, noise andvibration occur. However, due to suppression of the vaporization of theDME at the second and third gear trains 37 and 38, the noise and thevibration are suppressed.

[0062] (1-3) The shapes and the dimensions of the gear tooth 26 a and 32a of the gears 26 and 32 are same as those of the gear tooth 27 a and 33a of the gears 27 and 33 except for the facewidths h1 and h2. Therefore,common portions between the gears 26, 27, 32 and 33 are larger incomparison with setting the tooth depth of the gears 27 and 33 of thesecond-stage gear train 37 being shallower than that of the gears 26 and32 of the first-stage gear train 36 so as to set the theoreticaldischarge capacity D2 of the second-stage gear train 37 being smallerthan the theoretical discharge capacity D1 of the first-stage gear train36. Consequently, it is easy to manufacture the gears 26, 27, 32 and 33due to the common portions. Also, the relationship between the secondand third gear trains 37 and 38 is same.

[0063] (1-4) In the present preferred embodiment, the theoreticaldischarge capacity D2 of the second-stage gear train 37 is equal to theactual discharge capacity S1 of the first-stage gear train 36. Also, thetheoretical discharge capacity D3 of the third-stage gear train 38 isequal to the actual discharge capacity S2 of the second-stage gear train37. Therefore, the leak rate of the DME at the second-stage gear train37 is not relatively large. Also, the leak rate of the DME at thethird-stage gear train 38 is not relatively large. Meanwhile, theleakage of the DME at the second and third gear trains 37 and 38 causespower loss of the pump 1 that is converted into heat energy that raisesthe temperature of the DME. However, since the leak rate of the DME isnot relatively large, the power loss of the pump 1 is suppressed, andthe DME does not become hot relatively.

[0064] In the present preferred embodiment, as mentioned above, the pump1 copes with ensuring the pressurization characteristics of the DME andthe suppression of the noise and the vibration due to the suppression ofthe vaporization of the DME at the second and third gear trains 37 and38. Also, the pump 1 copes with the suppression of the power loss of thepump 1 and the increase in the temperature of the DME due to thereduction of the leak rate of the DME. Consequently, the pump 1 hasexcellent performance.

[0065] (1-5) The relief valve 46 is provided at the end portion of thefirst passage 43 a on the side of the second passage 43 b of thecommunication passage 43. When a required pressure of the pump 1 isvaried and the DME is pressurized to the required pressure only by thefirst-stage gear train 36, a part of the DME is discharged from therelief valve 46 into the discharge port 39 a. Therefore, it is avoidedthat the DME is not sucked into the second-stage gear train 37 and leaksto the first-stage gear train 36 when the DME has a sufficiently highpressure. Consequently, the power loss of the pump 1 can be suppressed.

[0066] Also, for example, when a clearance between the gears 26 and 32of the first-stage gear train 36 and the connecting plate 21 is smallerthan an assumed value due to variation in design dimensions of the gears26 and 32 of the first-stage gear train 36 and the connecting plate 21,only an unexpectedly small amount of the DME leaks at the first geartrains 36. Namely, the first-stage gear train 36 has a relatively highpressurization capacity. Therefore, even though the DME is pressurizedto the required pressure at the first-stage gear train 36 under theconditions, a part of the DME is discharged from the relief valve 46into the discharge port 39 a. As a result, workload of the second-stagegear train 37 is reduced. Also, since the relief valve is provided onthe communication passage 47, the relationship between the second andthird gear trains 37 and 38 is same.

[0067] (1-6) The O-rings 48 a through 48 f, the seal rings 49 a through49 f and the seal rings 50 a through 50 f are arranged in the gearassembly 11 for ensuring the sealing between the pump chambers 36 athrough 38 a and 36 b through 38 b and the interior of the casing 7.Therefore, the DME passing through the gear trains 36 through 38 is hardto leak out to the interior of the casing 7 and the internal chambers 51and 52.

[0068] (1-7) The driven shaft 29 is integrally formed with a single gear(the gear 32), and the other gears 33 and 34 are assembled to the drivenshaft 29 so as to be rotatable relative to the driven shaft 29.Therefore, the gears 33 and 34 are rotated by the drive shaft 12. It isavoided that only a single gear out of the gears 32 through 34 receivesload for the three gears 32 through 34. Consequently, decrease indurability of the gears 32 through 34 can be suppressed.

[0069] For example, assuming that the driven shaft 29 is fixed to thegear assembly 11 so as to function as a fixed axle and rotatablysupports the gears 32 through 34, the circumferential speed of the gears32 through 34 relative to the fixed axle is excessive. Therefore, inthis case, it is necessary to interpose bearings between the gears 32through 34 and the fixed axle so as to rotate the gears 32 through 34without trouble. However, interposing the bearings makes the pump 1large in size. Therefore, it is improper to function the driven shaft 29as the fixed axle. It is preferable that the driven shaft 29 rotates atthe substantially same rotational speed as the gears 32 through 34.

[0070] In the present preferred embodiment, even though a phase of thedriven shaft 29, which rotates integrally with the gear 32 engaged withthe gear 26, is different from a phase of the gear 33 engaged with thegear 27 and a phase of the gear 34 engaged with the gear 28, the drivenshaft 29 rotates at the substantially same rotational speed as the gears33 and 34. In the structure to rotate the driven shaft 29 at thesubstantially same rotational speed of the gears 33 and 34 withoutinterposing the bearings between the gears 33 and 34 and the drivenshaft 29, the driven shaft 29 is rotated integrally with the single gear(the gear 32). Therefore, the decrease in the durability of the gears 32through 34 is suppressed.

[0071] (1-8) The driven shaft 29 is formed integrally with the singlegear (the gear 32). For example, assuming that the gear 32 rotatesintegrally with the driven shaft 29 through a key, the bearing 31 needsto be arranged away from the key at a position where the bearing 31 doesnot interfere with the key (When a gear is connected to a shaft througha key, the length of the key is generally larger than the facewidth ofthe gear.). In the present preferred embodiment, since the gear 32 isformed integrally with the driven shaft 29 without a key, the bearing 31is arranged near the gear 32 in comparison to utilizing a key.Therefore, the driven shaft 29 is hard to bend.

[0072] A radial load resulting from the rotational torque and a radialload resulting from the pressure in the pump chambers 36 a through 38 aact on the gears 26 through 28 at the side of the drive shaft 12 in adirection in which the above radial loads cancel each other. Incontrast, the above radial loads act on the gears 32 through 34 at theside of the driven shaft 29 in a direction in which the above radialloads overlap each other. Therefore, the radial load on the gears 32through 34 at the side of the driven shaft 29 is larger than that on thegears 26 through 28 at the side of the drive shaft 12 (occasionallytwice). In this situation, the driven shaft 29 receiving the radial loadfrom the gears 32 through 34 bends easily.

[0073] Since the three gears 32 through 34 are formed or assembled tothe driven shaft 29, a span between the bearings 30 and 31, that is, abearing span is large in comparison to mounting a single gear or twogears on the driven shaft 29. Due to the large bearing span, the drivenshaft 29 bends easily in this situation. Therefore, the structure toform the gear 32 integrally with the driven shaft 29 and to arrange thebearing 31 near the gear 32 is effective in preventing the driven shaft29, which bends easily, from bending.

[0074] (1-9) Since the gear 32 having the largest facewidth among thegears 32 through 34 is formed integrally with the driven shaft 29, loadapplied from the drive shaft 12 to the gears 33 and 34 being rotatablerelative to the driven shaft 29 can be dispersed in comparison toforming the gear 33 or 34 integrally with the driven shaft 29.

[0075] (1-10) The return passage is formed in the gear assembly 11 andinterconnects the internal chamber 51 with the suction port 35 a. TheDME leaking out to the internal chamber 51 is returned to the suctionport 35 a through the return passage. Therefore, the pressure in theinternal chamber 51 is substantially equal to the suction pressure, andthe thrust load on the drive shaft 12 is hard to generate.

[0076] (1-11) The port 56 is formed in the pipe connecting portion 54 soas to interconnect the internal space of the motor 10 with the outsideof the pump 1. The drain hole 57 is formed in the sidewall of the motorhousing 10 a so as to interconnect the internal space of the motor 10with the space that is inside the casing 7. Therefore, the DME thatevaporates and that is stored inside the casing 7 and the motor housing10 a is returned from the port 56 and the drain hole 57 into the tank 2through the pipe 55. Consequently, insufficient coupling of the motor 10caused due to congestion of the DME gas does not occur.

[0077] (1-12) The pump 1 includes the motor 10 and is a pump that sealsa shaft therein. Therefore, an external drive source is unnecessary whenthe pump 1 is operated.

[0078] Next, the second preferred embodiment will be described accordingto FIG. 6. A pump 1 of the second preferred embodiment is a two-stagegear pump from which a shaft 12 protrudes. In the second preferredembodiment, only the difference between the first and second preferredembodiment is described, and the identical or corresponding members arereferred to the same reference numbers as those in the first preferredembodiment.

[0079]FIG. 6 is a cross-sectional view of the pump 1. A casing 7 of thepresent preferred embodiment is open to the lower side (the right sidein FIG. 6). A cover 9 is secured to the lower end of a casing 7. Thedrive shaft 12 protrudes from the upper side of the casing 7 to theoutside of the casing 7, and the protrusion of the drive shaft 12 isconnected to an external drive source (not shown). The pump 1 of thepresent preferred embodiment is the two-stage gear pump including afirst-stage gear train 36 and a second-stage gear train 37. Thefirst-stage gear train 36 is arranged on the upper side, and thesecond-stage gear train 37 is arranged on the lower side.

[0080] A base block 16, a connecting plate 21, a side plate 20 and aconnecting plate 19 are accommodated in the casing 7 so as to contacteach other. In the present preferred embodiment, a gear 32 constitutingthe first-stage gear train 36 is formed separately from a driven shaft29. A gear 33 constituting the second-stage gear train 37 as thehighest-pressure stage gear train is formed integrally with the drivenshaft 29. A discharge connecting portion 39 is provided on the cover 9,and a suction connecting portion 35 is not shown in the drawings.

[0081] In the present preferred embodiment, the same advantageouseffects are obtained as mentioned in paragraphs (1-1) through (1-8)according to the first preferred embodiment. Also, followingadvantageous effects are obtained.

[0082] (2-1) The gear 33 constituting the highest-pressure stage geartrain (the second-stage gear train 37) is formed integrally with thedriven shaft 29. The gear 33 of the second-stage gear train 37 has afacewidth that is smaller than that of 5 the gear 32 constituting thefirst-stage gear train 36. Namely, the thickness of the gear 33 issmaller than that of the gear 32. As the thickness of a gear is small,the gear is easily inclined with respect to the driven shaft 29.However, since the gear 33 whose thickness is smaller than that of thegear 32 is formed integrally with the driven shaft 29, the aboveinclination does not occur. Therefore, in comparison to to forming thegear 32 with a large thickness integrally with the driven shaft 29,seizure and abrasion due to the inclination of the gear 32 areeffectively avoided.

[0083] When the gears 26 and 27 are inclined with respect to the driveshaft 12, a component force of the radial load occurs in the gears 26and 27 in the axial direction. Similarly, when the gears 32 and 33 areinclined with respect to the driven shaft 29, a component force of theradial load occurs in the gears 32 and 33 in the axial direction. Asmentioned above, the radial load at the side of the driven shaft 29 islarger than that at the side of the drive shaft 12. Therefore, thecomponent force in the axial direction in the gears 32 and 33 at theside of the driven shaft 29 is larger than that in the gears 26 and 27at the side of the drive shaft 12, and the seizure and the abrasion dueto the inclination of the gears 32 and 33 occurs more easily. In thepresent preferred embodiment, since the gear 33 having a facewidth thatis smaller than that of the gear 32 at the side of the driven shaft 29is formed integrally with the driven shaft 29, avoiding the inclinationof the gear 33 with respect to the driven shaft 29 suppresses thecomponent force in the axial direction. Consequently, in the pump 1, theseizure 5 and the abrasion due to the inclination of the gears 26 and 27and 32 and 33 with respect to the drive shaft 12 and the driven shaft 29are effectively avoided.

[0084] (2-2) Chamfered portions 32 b are formed at connecting portionsbetween the side surfaces and the inner circumferential surface in thegear 32 so as to assemble the driven shaft 29 through the gear 32easily. Also, chamfered portions are formed in the gears 26 and 27 andare not shown in the drawings. For example, it is assumed that the gear32 is formed integrally with the driven shaft 29 and that the drivenshaft 29 is inserted through the gear 33, chamfered portions with thesame radii of curvature as the chamfered portion 32 b are formed in thegear 33. In the present preferred embodiment, a rate of the width of thechamfered portions 32 b relative to the facewidth of the gear 32 in theinner circumferential surface of the gear 32 is small in comparison tothe above-assumed case. Therefore, since the gear 32 is harder toincline than the gear 33 and the driven shaft 29 is inserted through thegear 32, the seizure and the abrasion due to the inclination of the gearare effectively avoided. Also, increase in contact pressure ofcontacting portions (the inner circumferential surface of the gear 32 inthe present preferred embodiment) in the gear 32 at the side of thedriven shaft 29 can be suppressed, and the durability of the gears 32and 33 can be improved. The radial load at the side of the driven shaft29 is larger than that on the side of the drive shaft 12 as mentionedabove and acts on the contacting portion of the gear 32. Therefore, theincrease in the contact pressure of the contacting portion of the gear32 is suppressed more effectively.

[0085] (2-3) The gear 33 constituting the second-stage gear train 37 asthe highest-pressure stage gear train is formed integrally with thedriven shaft 29. Therefore, the axis of the outer circumference of thegear 33 is offset relative to the axes of the gear 29 and the gearassembly 11 in a relatively small degree, so a leak rate of the DME atthe high-pressure stage gear train (the second-stage gear train 37) isrelatively small. Since the discharge capacity of the high-pressurestage gear train is set to be smaller than that of the low-pressurestage gear train (the first-stage gear train 36), the leak rate of theDME affect volume efficiency of the high-pressure stage gear train.However, the leak rate of the DME can be reduced, and high volumeefficiency can be maintained.

[0086] (2-4) The pump 1 is a pump from which a shaft protrudes and thatis driven by an external drive source. There is not a motor in the pump1. Therefore, the pump 1 can be small.

[0087] The preferred embodiment according to the present invention isnot limited to the above preferred embodiments and may be changed tofollowing alternative embodiments.

[0088] In the first preferred embodiment, the gear 32 of the first-stagegear train 36 is formed integrally with the driven shaft 29. However,the gear 33 of the second-stage gear train 37 or the gear 34 of thethird-stage gear train 38 may be formed integrally with the driven shaft29. For example, in the first preferred embodiment, the gear 34constituting the third-stage gear train 38 is formed integrally with thedriven shaft 29, and the gears 32 and 34 as other gears are assembled tothe driven shaft 29 so as to be rotatable relative to the driven shaft29. The pump 1 in the first preferred embodiment has three gear trainswhile the pump 1 in the second preferred embodiment has two gear trains.Therefore, in this case, the substantially same advantageous effects areobtained notably as mentioned in the paragraphs (2-1) through (2-3)according to the second preferred embodiment.

[0089] For example, corresponding to the effect mentioned in paragraph(2-1), as the facewidth of the gear becomes small, the thickness of agear is smaller, and the gear with the relatively small thickness iseasy to incline with respect to the driven shaft 29 (when the gear isassembled to the driven shaft 29 so as to be rotatable relative to thedriven shaft 29.). The gear 34 of the third-stage gear train 38 mosteasily inclines with respect to the driven shaft 29. However, since thegear 34 is formed integrally with the driven shaft 29, the seizure andthe abrasion due to the inclination of the gear 34 with respect to thedriven shaft 29 are effectively avoided. Consequently, regarding onlyobtaining the effect mentioned in paragraph (2-1) effectively, as thenumber of the gear trains of a pump increases: the structure to form agear constituting a highest-pressure stage gear train integrally withthe driven shaft 29 is effective, especially for a gear pump having morethan three gear trains.

[0090] In the second preferred embodiment, the gear 33 of thesecond-stage gear train 37 is formed integrally with the driven shaft29. However, the gear 32 of the first-stage gear train 36 may be formedintegrally with the driven shaft 29. In this case, the substantiallysame advantageous effect is obtained as mentioned in paragraph (1-9)according to the first preferred embodiment.

[0091] In the first preferred embodiment, the relief valve (not shown)and the relief valve 46 as the valve means are arranged in the sideplates 18 and 20 respectively. A relief valve may be arranged in one ofthe side plates 18 and 20. Also, the valve means is not an internalautonomous type as the relief valve 46, the valve means may be anexternal control type valve (e.g. an electromagnetic valve) that opensand closes based on an output of a sensor that detects whether or notthe pressure of the DME discharged into the communication passage 43exceeds a predetermined value. The valve means may be removed from thepump 1.

[0092] In the first preferred embodiment, the relief valve 46 in an openstate is discharged the DME which has been pressurized to the requiredpressure into the discharge port 39 a. However, the DME is notdischarged from the relief valve 46 into the discharge port 39 a and maybe discharged from the relief valve 46 by bypassing the high-pressurestage gear train. For example, the DME may be discharged from the reliefvalve 46 into the downstream passage at the third-stage gear train 38.Also, the DME is not discharged from the relief vale 46 into theinterior of the pump 1 and may be discharged from the relief valve 46into the discharge pipe 4.

[0093] In the first preferred embodiment, a return passage thatinterconnects the first chamber 51 a with the third chamber 51 c and areturn passage that interconnects the second chamber 51 b with the thirdchamber 51 c may be provided. The DME leaking out to the first andsecond chambers 51 a and 51 b is sent to the third chamber 51 c throughthese return passages more smoothly than only through the slightclearance between the key 25 and the groove 12 a. Sequentially, the DMEis returned from the third chamber 51 c to the suction port 35 a throughthe already described return passage that interconnects the thirdchamber 51 c with the suction port 35 a.

[0094] Similarly, in the second preferred embodiment, a return passagethat interconnects the first chamber 51 a with the second chamber 51 bmay be provided. Therefore, the DME leaking out to the first chamber 51a is smoothly sent to the second chamber 51 b through the returnpassage. Sequentially, the DME is returned from the second chamber 51 bto a suction port through a return passage that interconnects the secondchamber 51 b with the suction port.

[0095] In the second preferred embodiment, the pump 1 is installed inthe vehicle in such a manner that the drive shaft 12 is arranged in avertical direction.

[0096] When an engine is utilized as an external drive source, the pump1 may be installed in the vehicle in such a manner that the drive shaft12 is arranged iii a horizontal direction.

[0097] The fluid that the multistage gear pump deals with is not theDME, and the present invention may be applied to a multistage gear pumpthat deals with other fluids. The present invention is effectiveespecially for a multistage gear pump that deals with fluid having oneof low viscosity characteristics and easily vaporized characteristics.

[0098] The present invention is applied to the three-gear pump 1 in thefirst preferred embodiment and the two-gear pump 1 in the secondpreferred embodiment. However, the present invention is not limited tothe above preferred embodiments and may be applied to multistage gearpumps, such as a four-gear or five-gear pump, other than the two-gearand three-gear pumps.

[0099] In the relationship between the first and second gear trains 36and 37, the facewidth h2 of the second-stage gear train 37 is set to besmaller than the facewidth h1 of the first-stage gear train 36, therebythe discharge capacity D2 of the second-stage gear train 37 is set to besmaller than the discharge capacity D1 of the first-stage gear train 36.However, the tooth depth of the gears 27 and 33 constituting thesecond-stage gear train 37 may be set to be shallower than that of thegears 26 and 32 constituting the first-stage gear train 36, thereby thedischarge capacity D2 of the second-stage gear train 37 is set to besmaller than the discharge capacity D1 of the first-stage gear train 36.Also, the same is true of the relationship between the second and thirdgear trains 37 and 38.

[0100] Although the O-rings 48 a through 48 f and the seal rings 49 athrough 49 f and 50 a through 50 f are provided in the first preferredembodiment and the O-rings 48 a through 48 d, the seal rings 49 athrough 49 d and 50 a through 50 d are provided in the second preferredembodiment, these O-rings and seal rings may be removed from the pump 1.

[0101] In the first and second preferred embodiments, the pump 1 is apump that pressurizes and sends the DME to the engine 6 for the vehicle.However, for example, the pump 1 may be a pump that pressurizes andsends operating oil to a machine tool.

[0102] In the first preferred embodiment, the gear 32 is formedintegrally with the driven shaft 29. However, the gear 32 may berotatable integrally with the driven shaft 29 through a key. Similarly,although the gear 33 is formed integrally with the driven shaft 29 inthe second preferred embodiment, the gear 33 may be rotatable integrallywith the driven shaft 29 through a key. In these cases, thesubstantially same advantageous effect is obtained as mentioned inparagraph (1-7) according to the first preferred embodiment.

[0103] Therefore, the present examples and embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein but may be modified within thescope of the appended claims.

What is claimed is:
 1. A multistage gear pump for pressurizing fluid,comprising: a housing; a drive shaft rotatably supported in the housing;and a gear assembly disposed in the housing, the gear assembly includingat least first and second gear trains, each train having a pair of driveand driven gears that are engaged with each other, the drive gear beingprovided on the drive shaft and followed by the driven gear, the firstgear train and the second gear train being arranged so that fluidsequentially passes therethrough as the drive shaft rotates, wherein atheoretical discharge capacity of the first gear train is larger thanthat of the second gear train.
 2. The multistage gear pump according toclaim 1, wherein the first gear train has a first facewidth and thesecond gear train has a second facewidth that is smaller than the firstfacewidth, thereby the theoretical discharge capacity of the second geartrain is set to be smaller than that of the first gear train.
 3. Themultistage gear pump according to claim 1, wherein the theoreticaldischarge capacity of the second gear train is set to be equal to anactual discharge capacity of the first gear train by considering a leakrate of the fluid.
 4. The multistage gear pump according to claim 3,wherein the actual discharge capacity of the first gear is determined tobe 70 to 80 percentage of the theoretical discharge capacity of thefirst gear train.
 5. The multistage gear pump according to claim 3,wherein the theoretical discharge capacity of the second gear train isequal to the actual discharge capacity of the first gear train with aslight tolerance.
 6. The multistage gear pump according to claim 5,wherein a maximum tolerance is 10 percentage of the theoreticaldischarge capacity of the first gear train.
 7. The multistage gear pumpaccording to claim 6, wherein the maximum tolerance is 5 percentage ofthe theoretical discharge capacity of the first gear train.
 8. Themultistage gear pump according to claim 1, wherein a valve means isprovided on a passage that interconnects the second gear train with thefirst gear train, the valve means opening so as to discharge the fluidin the passage by bypassing the second gear train when a pressure of thefluid in the passage exceeding a predetermined pressure.
 9. Themultistage gear pump according to claim 1, further comprising a drivenshaft, the driven gears being connected to the driven shaft, one of thedriven gears being formed integrally with the driven shaft, the rest ofthe driven gears being rotatable relative to the driven shaft.
 10. Themultistage gear pump according to claim 9, wherein the first gear trainhas a first facewidth and the second gear train has a second facewidththat is smaller than the first facewidth, the driven gear constitutingthe gear train whose pressure is the highest of the gear trains beingformed integrally with the driven shaft.
 11. The multistage gear pumpaccording to claim 9, wherein the first gear train has a first facewidthand the second gear train has a second facewidth that is smaller thanthe first facewidth, the driven gear which is formed integrally with thedriven shaft constituting a first gear train through which the fluid ispassed firstly.
 12. The multistage gear pump according to claim 1,wherein the fluid is dimethylether.
 13. The multistage gear pumpaccording to claim 1, wherein the fluid has at least one of lowviscosity characteristics and easily vaporized characteristics in aliquid state.
 14. A multistage gear pump for pressurizing fluid,comprising: a housing; a drive shaft rotatably supported in the housing;a driven shaft rotatably supported in the housing; and a gear assemblydisposed in the housing, the gear assembly including at least first andsecond gear trains, each train having a pair of drive and driven gearsthat are engaged with each other, the drive gear being provided on thedrive shaft and followed by the driven gear, the first gear train andthe second gear train being arranged so that fluid sequentially passestherethrough as the drive shaft rotates, wherein one of the driven gearsis formed with the driven shaft so as to rotate integrally with thedriven shaft, the rest of the driven gears being assembled to the drivenshaft so as to rotate relative to the driven shaft.
 15. The multistagegear pump according to claim 14, wherein the driven gear that isrotatable integrally with the driven shaft is formed integrally with thedriven shaft.
 16. The multistage gear pump according to claim 15,wherein the facewidths of the gear trains are different from each other,the driven gear which is formed integrally with the driven shaftconstituting a gear train that has a smallest facewidth.
 17. Themultistage gear pump according to claim 15, wherein the facewidths ofthe gear trains are different from each other, the driven gear which isformed integrally with the driven shaft constituting a gear train thathas a largest facewidth.